Reuse Site Columns in SharePoint 2010
You've probably already been using site columns. All the standard SharePoint 2010 lists — Calendar, Contacts, Announcements, and so on — use site columns. That’s why each time you create a new Announcements list, you always get the same columns every single time.
Instead of creating your own site columns, you can reuse the site columns provided by SharePoint. Site columns are stored in the Site Column gallery. The Site Column gallery is accessible from the Site Settings page in your site collection. You can browse around this gallery and see what site columns already exist.
Sometimes the site columns that Microsoft provides are intended for use that’s different than the name implies. For example, the Active Site column pertains to document routing. The actual internal name of the column is RoutingEnabled. Sometimes creating your own column is better than reusing Microsoft’s columns.
Site columns in content types is a great way to group several columns in a meaningful (and reusable) way. You can also associate site columns directly with lists and libraries using the List Settings or Library Settings page of your list or library.
Click the Add from Existing Site Columns link to add your custom site columns or the site columns provided by SharePoint to the list of columns that are available to your list or library.